Mobile communication systems employ a Hybrid Automatic Repeat reQuest (HARQ) technique that is a physical layer packet retransmission algorithm in order to increase a downlink or uplink throughput. In a description of a downlink as an example, in a system employing the HARQ technique, a mobile station (MS) determines if packet reception is success or failure through an error check for a packet received from a base station (BS), and sends an Acknowledgement or Negative Acknowledgement (ACK or NACK) response depending on the determination result. Upon receiving the ACK, the BS transmits a next packet (i.e., a new packet) to the MS. Upon receiving the NACK, the BS retransmits a previous packet to the MS. At this time, the MS stores a previously received packet in a buffer. When the same packet is retransmitted by the BS, the MS combines the retransmitted packet with the previous packet and demodulates the combined data. As such, the HARQ technique has an advantage of increasing the probability of successful packet reception, thus improving a link throughput.
A transmission rate is controlled in order to satisfy a Carrier to Interference and Noise Ratio (CINR) value that is adaptive to changing channel conditions. A system has a plurality of link tables optimized to a movement speed and a surrounding environment. The BS determines a transmission method according to a CINR (i.e., a Channel Quality Indicator (CQI)) received from the MS, using each different link table depending on a channel model. The link table is a table predefining a data transmission method based on the CINR. The link table defines a CINR value that satisfies a Packet Error Rate (PER) of 1% at each Modulation and Coding Scheme (MCS) level according to a channel change.
FIG. 1 illustrates a signal flow between a BS and an MS for determining an MCS level according to the conventional art.
Referring to FIG. 1, in step 101, a BS 110 sends a request for CQI to an MS 120. If so, in step 103, the MS 120 estimates a downlink channel using a pilot signal or preamble signal, etc., received from the BS 110. In step 105, the MS 120 reports the estimated CQI to the BS 110. The CQI can be a channel coefficient or the CINR. Then, in step 107, the BS 110 determines a suitable MCS level through a comparison between the CINR, etc., reported from the MS 120 and the link table. Then, in step 109, the BS 110 modulates downlink data at the determined MCS level and transmits the modulated data to the MS 120.
The link table is expressed by a CINR critical value for each MCS level. The critical value is determined experimentally on the assumption of a plurality of channel conditions. Thus, the link table does not perfectly reflect all conditions in which a system actually operates. If a difference between the link table and actual system operation conditions is generated, transmission efficiency may reduce although an MCS level is determined using a CINR measured by the MS. In order to solve this problem, the BS can apply an Outer-Loop Rate Control (OLRC).
FIG. 2 illustrates a signal flow between a BS and an MS for determining an MCS level using OLRC according to the conventional art.
Referring to FIG. 2, in step 201, a BS 110 sends a request for CQI to an MS 120. If so, in step 203, the MS 120 estimates a downlink channel using a pilot signal or preamble signal, etc., received from the BS 110. In step 205, the MS 120 reports the estimated CQI to the BS 110. The CQI can be a channel coefficient or the CINR. Then, in step 207, the BS 110 determines a suitable MCS level through a comparison between the CINR, etc. reported from the MS 120 and the link table. Then, in step 209, the BS 110 modulates downlink data at the determined MCS level and transmits the modulated data to the MS 120.
In step 211, the MS 102 performs an error check for downlink data received from the BS 110 and determines if reception is succeeded or failed depending on the error check result. In step 213, the MS 120 feeds back an ACK or NACK to the BS 110 depending on the success or failure reception. When reception is succeeded, the MS 120 feeds back the ACK and, when reception is failed, feeds back the NACK. In step 215, the MS 120 estimates a channel and, in step 217, transmits the estimated CQI to the BS 110.
Then, in step 219, the BS 110 performs OLRC using the feedback signal (i.e., the ACK or NACK), the CINR, etc.
In step 221, for an MS supporting HARQ, the BS 110 can determine an MCS level using a CQI and HARQ ACK/NACK message fed back from the MS. However, for an MS not supporting HARQ (hereinafter, referred to as “non-HARQ MS”), the BS 110 has to determine an MCS using only a CQI. Thus, OLRC cannot be applied. Therefore, a difference with system operation conditions is generated, causing a problem of reducing transmission efficiency.